【EmguCV】C#實現HOG與SVM的幾個問題，

【EmguCV】C#實現HOG與SVM的幾個問題，

關於SVM中的alpha、rho向量

由於EmguCV封裝的更加徹底，在C#中並不能跟C++一樣通過重載獲得這兩個中間變量

//繼承自CvSVM的類，因為生成setSVMDetector()中用到的檢測子參數時，需要用到訓練好的SVM的decision_func參數，  
//但通過查看CvSVM源碼可知decision_func參數是protected類型變量，無法直接訪問到，只能繼承之後通過函數訪問  
class MySVM : public CvSVM  
{  
public:  
    //獲得SVM的決策函數中的alpha數組  
    double * get_alpha_vector()  
    {  
        return this->decision_func->alpha;  
    }  
  
    //獲得SVM的決策函數中的rho參數,即偏移量  
    float get_rho()  
    {  
        return this->decision_func->rho;  
    }  
};

見C++實例：訓練SVM分類器進行HOG行人檢測 http://blog.csdn.net/pb09013037/article/details/41256945）

為了獲取這兩個變量用於自定義HOG檢測子，暫時想到的幾種辦法：

1、C#讀取生成的XML文件

分類器訓練好後一般需要進行保存，方便直接預測

SVM svm = new SVM();
bool trained = svm.Train(my_train.sampleFeatureMat, my_train.sampleLabelMat, null, null, p);
svm.Save(@"../HOG_SVM.xml");

這裡給出我的C#提取SVM參數方式：
（只用於提取訓練目標為1與-1兩類的XML文件，如果類型大於2，則有多個rho與alpha數組，需要進一步組合）

using System;
using System.Text;
using System.Xml;
using System.IO;


namespace HOG_SVM
{
    class GetData
    {
        public double[] alpha;
        public double rho;

        XmlDocument doc;        
        StreamReader sr;
        int sv_count;
        string alpha_str;

        public GetData()
        {
            doc = new XmlDocument();
            doc.Load(Form1.LOAD_PATH);
            XmlNode nodes = doc.DocumentElement;
            get_rho(nodes);
            getAlpha_str(nodes);
            getSv_count(nodes);
            getAlpha(); 
        }

        public void get_rho(XmlNode nodes)
        {
            if (nodes.HasChildNodes)
            {
                foreach (XmlNode node in nodes.ChildNodes)
                {
                    if (nodes.Name == "rho")
                    {
                        rho = Double.Parse(nodes.InnerText);
                        return;
                    }
                    get_rho(node);
                }
            }
        }

        public void getAlpha_str(XmlNode nodes)
        {
            if (nodes.HasChildNodes)
            {
                foreach (XmlNode node in nodes.ChildNodes)
                {
                    if (nodes.Name == "alpha")
                    {
                        //sr = new StreamReader(new Stream(nodes.InnerText));
                        alpha_str = nodes.InnerText;
                        return;
                    }
                    getAlpha_str(node);
                }
            }
        }

        public void getSv_count(XmlNode nodes)
        {
            if (nodes.HasChildNodes)
            {
                foreach (XmlNode node in nodes.ChildNodes)
                {
                    if (nodes.Name == "sv_count")
                    {
                        sv_count = int.Parse(nodes.InnerText);
                        return;
                    }
                    getSv_count(node);
                }
            }
        }
        
        public void getAlpha()
        {
            byte[] array = Encoding.ASCII.GetBytes(alpha_str);
            MemoryStream stream = new MemoryStream(array);             //convert stream 2 string      
            sr = new StreamReader(stream);
            alpha = new double[sv_count];
            sr.ReadLine();
            int i = 0;
            while (true)
            {

                string tmp = sr.ReadLine();
                if (tmp == "")
                    continue;

                string[] tmp2 = tmp.Split(' ');
                foreach (string ele in tmp2)
                {
                    if (ele != "")
                    {
                        alpha[i] = double.Parse(ele);
                        i++;
                    }
                }

                if (i == sv_count)
                    break;
            }
        }        
    }
}

c#讀取XML的方式比較多，還可以利用Linq操作xml，另外也可以參考以下鏈接：
c# 讀取opencv 生成的svm訓練好的xml分類器：http://blog.csdn.net/yeyang911/article/details/12905153

2、使用其他C#的SVM庫

關於提取參數，自定義HOG Detector的問題，後來在網上搜到了這種方式
Training custom SVM to use with HOGDescriptor in OpenCV：




I was struggling with the same problem. Searching forums I have found, that the detector cannot be trained using CvSVM (I don't know the reason). I used LIBSVM for training the the detector. Here is the code to extract the detector for HOGDescriptor.setSVMDetector( w): For data details see LIBSVM documentation/header. I did all the training in C++, filling the LIBSVM training data from CV to LIBSVM; the code below extracts the detector vector needed for cv::HOGDescriptor. The w parameter is std::vector<float> w    





const double * const *sv_coef = model.sv_coef;
const svm_node * const *SV = model.SV;
int l = model.l;
model.label;

const svm_node* p_tmp = SV[0];
int len = 0;
while( p_tmp->index != -1 )
{
    len++;
    p_tmp++;
}
w.resize( len+1 );

for( int i=0; i<l; i++)
{
    double svcoef = sv_coef[0][i];
    const svm_node* p = SV[i];
    while( p->index != -1 )
    {
        w[p->index-1] += float(svcoef * p->value);
        p++;
    }
}
w[len] = float(-model.rho[0]);





來自: http://stackoverflow.com/questions/15339657/training-custom-svm-to-use-with-hogdescriptor-in-opencv

該回答提到的 LIBSVM 庫就是比較好的替代手段，應該可以直接獲取到這兩個中間量，而不用再去解析XML。
可以去作者主頁上下載LIBSVM庫：http://www.csie.ntu.edu.tw/~cjlin/libsvm/#csharp

3、其他相關鏈接

前些天的【OpenCV】基於HOG與SVM的行人檢測學習（原理小結）:


    http://www.cnblogs.com/KC-Mei/p/4534009.html


training GPU HOGDescriptor for multi scale detection：


    http://answers.opencv.org/question/4351/training-gpu-hogdescriptor-for-multi-scale-detection/